1. Field of the Invention
The present invention relates to a steelmaking process and, more particularly, a steelmaking process comprising a series of refining steps for converting the molten pig iron obtained from a blast furnace into molten steel.
2. Description of the Prior Art
Recently, in accordance with the development of ultra low sulfur steels and ultra low phosphorus steels, stricter demands are imposed upon the dephosphorization and desulfurization of the steelmaking process. In the conventional steelmaking process, most of the impurities such as silicon, phosphorus, sulfur and carbon are removed in the blowing step using a converter, with the result that the load, which the converter must bear in the steelmaking operation, becomes high. According to a known process which aims to mitigate the converter load and to simplify the control of each component of the molten iron, several impurities are removed at the pig iron stage, while in the converter mainly decarburization is carried out. An example of the known process mentioned above is that disclosed in Japanese Laid Open Patent Application 127421/1977, wherein the desiliconization is carried out by an iron oxide or oxygen, followed by a simultaneous dephosphorization and desulfurization by means of Na.sub.2 CO.sub.3. The removal of all the silicon, phosphorus and sulfur in the pig iron stage is desirable from the view point of mitigating the converter load. However, from the view point of the desulfurization and dephosphorization reactions, the desulfurization treatment is desirably realized under a reducing atmosphere i.e. with a slag having low FeO content, while the dephosphorization treatment is desirably realized under an oxidizing atmosphere, i.e. with a slag having high FeO content. Efficient desulfurization and dephosphorization conditions are, therefore, contradictory to one another. Accordingly, simultaneous desulfurization and dephosphorization are not efficient and thus involve problems when applied in practical operation.
The two kinds of refining agents mentioned hereinafter are mainly used at present for the simultaneous desulfurization and dephosphorization. Namely, one of the refining agents is based on Na.sub.2 CO.sub.3, while the other is based on CaO and an oxidizer, such as a mill scale, iron ore, oxygen gas and the like. As illustrated in Japanese Laid Open patent application No. 127421/1977, Na.sub.2 CO.sub.3 is an efficient flux for the simultaneous desulfurization and dephosphorization of a low silicon-molten pig iron, because Na.sub.2 CO.sub.3 has within itself "O", which is an oxidizer, and "Na.sub.2 O" which is a base. In the dephosphorization reaction, the reaction between O, Na.sub.2 O and P formulated as: EQU 5O (from Na.sub.2 CO.sub.3)+2P+3Na.sub.2 O.fwdarw.3Na.sub.2 O.P.sub.2 O.sub.5,
proceeds, while in the desulfurization reaction, the reaction between Na.sub.2 O and S formulated as: EQU Na.sub.2 O+S.fwdarw.Na.sub.2 S+O,
proceeds. The processing unit of Na.sub.2 CO.sub.3 described in the Japanese Laid Open Patent Application is in the range of from 10 to 60 kg/t. The use of Na.sub.2 CO.sub.3 as the refining agent or flux involves problems from the view points of excessive cost and erosion of the refractory of the processing vessel due to vigorous reactivity of Na.sub.2 CO.sub.3 as well as environmental pollution due to formation of smoke and fumes. The flux based on Na.sub.2 CO.sub.3 is, therefore, not suitable for practical application for the desulfurization and dephosphorization.
Also, with regard to the simultaneous desulfurization and dephosphorization by means of the refining agent based on the oxidizer and CaO, effective desulfurization and dephosphorization conditions are contradictory to one another as explained hereinabove, and, an excess CaO is necessary to carry out the desulfurization under an oxidizing atmosphere or under the presence of the oxidizer. The simultaneous desulfurization and dephosphorization are therefore of low efficiency, and, therefore the desulfurization and dephosphorization processes should be carried out in two separate stages.
Incidentally, silicon, phosphorus and sulfur are desirably removed at the molten pig iron stage, and various proposals have been made with regard to the removal of silicon and the like. However, if three stages for desiliconization, dephosphorization and desulfurization, respectively, are employed in the processing of the pig iron, not only does the steelmaking process become complicated but also the temperature drop of molten pig iron during the processing is so conspicuous, that the industralization of this process with the three stages becomes difficult.
Since the removal and shape-control of the non-metallic inclusions have recently been required to meet the stricter demands for producing clean steels, development of a secondary refining process after the steel tapping, such as an inert-gas blowing and degassing, is promoted. The desulfurization, desiliconization and dephosphorization described hereinabove are carried out separately or a plurality of them occur continuously or simultaneously in the previous various proposals. However, a process for treating all impurities of molten iron, wherein the individual divided steps are combined systematically so as to provide an efficient refining technique, has not yet been proposed.